docs(#2505): Phase 6 — migration guide + built-in-only subagent-toolkit enum (#2538)

* docs(#2512): Phase 6 — migration guide + built-in-only subagent-toolkit enum

* fix #2512: update CONTRACT-PIN for built-in-only subagentToolkit value

* docs(changeset): backfill PR #2538 for Phase 6 (#2512)

capabilities/kimi-code/capability.json

@@ -58,7 +58,7 @@
         "nested": false,
         "maxDepth": 1,
         "background": true,
-        "subagentToolkit": "undocumented",
+        "subagentToolkit": "built-in-only",
         "backgroundDispatch": true,
         "builtInSubagents": [
           "coder",